identity: fix another principal leak
identity: fix another principal leak

Spotted by valgrind.

https://bugzilla.gnome.org/show_bug.cgi?id=729865

Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,felipeborges/gnome-online-accounts,halfline/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,dnohales/gnome-online-accounts,ritzk/gnome-online-accounts,halfline/gnome-online-accounts,Distrotech/gnome-online-accounts,felipeborges/gnome-online-accounts,ritzk/gnome-online-accounts,felipeborges/gnome-online-accounts,Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts